This invention relates generally to the fabrication of resistive elements and more particularly to a thick film process therefor. Such a process might be employed, for example, in constructing a thermal print head containing a plurality of resistive elements which act to produce marks upon an adjacent thermally-sensitive record material upon energization by a short duration pulse of electric current. A print head so constructed might be employed as shown and described in a copending U.S. Patent Application Ser. No. 250,703, now abandoned entitled THERMAL PRINTER, filed by Robert E. McMillan, Jr. May 5, 1972, and assigned to the same assignee as this application. Resistive elements constructed according to the prior art have typically been applied to a semiconductor substrate by conventional thin film processes. The result is a thin element which is susceptible to abrasion by the thermal record material passing the print head. This abrasion over a period of time effects changes in the basic characteristics of the resistors which result in deteriorated printing quality. In addition, the resistive elements have typically been applied to an edge surface of the substrate, while their respective conductors have been applied to the face surfaces of the substrate. This arrangement is not feasible in applications requiring a large number of resistive elements because of the small area available on the substrate edge for making interconnections between the elements and their conductors. Also, considerable difficulty has been encountered in controlling the surface of resistors which are applied to the edge of a substrate. For these reasons it would be advantageous to position both the resistive elements and their conductors on one face of the substrate. Another problem exists because the elements have been attached directly to the substrate. Upon energization, a considerable portion of the applied power escapes into the substrate, thus producing less heat in the element itself. Because of this inefficiency greater amounts of power are required to perform the printing operation.
In fabricating resistors as part of multilayer thick film circuits, it has previously been possible to screen print them only directly on the substrate without adversely affecting the resistance, temperature coefficient of resistance, long term stability, and power handling capability of the resistive material. These prior art techniques allowed only those resistance and temperature coefficient of resistance values yielded by the basic material so that special requirements could not be met. It is desirable to be able to screen print resistors over a glass layer to enable placement thereof at any layer of a multilayer circuit. It is also desirable to be able to tailor the various resistors and their respective temperature coefficients of resistance without affecting resistor power handling capability.
Accordingly, it is an object of this invention to provide an improved process of fabricating thick film resistive elements such as might be used, for example, in thermal print heads.
It is another object of this invention to provide an improved process for fabricating thick film resistive elements on top of a layer of glass material.
It is a further object of this invention to provide an improved process for fabricating thick film resistive elements having resistance and temperature coefficient of resistance values that may be tailored to particular requirements.
These objects are accomplished in accordance with the preferred embodiment of this invention by depositing a layer of glass material upon a ceramic substrate using screen printing techniques well known in the art. The glass material is then fired in air at an elevated temperature. A resistor-conductor network is then placed thereon, again using conventional screen printing processes. The resistors are next fired, lapped, and annealed.